(MAINT) Fix checkout counter allocation

Checkout metric counters were against the template name and not the
actual pool used which prevents us from counting the checkouts in the
pixa4 pools for example.

lib/vmpooler/api/v1.rb

@@ -209,7 +209,7 @@ module Vmpooler
             break
           else
             vms << [vmpool, vmname, vmtemplate]
-            metrics.increment("checkout.success.#{vmtemplate}")
+            metrics.increment("checkout.success.#{vmpool}")
           end
         end
       end